(1) Field of the Invention
The present invention generally relates to an optical communication in which a light signal indicating digital data is transmitted through an optical transmission line, and more particularly to an optical transmitter in which light is modulated in accordance with digital data and a light signal is output to an optical transmission line. Also, the present invention relates to a laser diode module which is suitable for use in the optical transmitter.
(2) Description of the Related Art
In optical communication, a large quantity of data can be transmitted at a high speed. Currently, optical communication is being applied practically in many industries. Because frequencies of light signals used in the optical communication are extremely high, an intensity modulation is widely used for optical transmitters to modulate light in accordance with digital data.
Generally, an optical transmitter employs a laser diode or some other semiconductor laser as the light source, and light from the laser diode is modulated in accordance with a data signal indicating digital data so that a light signal carrying the data is produced. The light signal is output by the optical transmitter to an optical transmission line for the data transmission.
There is a problem that the output of the laser diode continuously oscillates if an excessive current greater than a threshold current flows in the laser diode. Also, the laser diode has a problem that the input-output characteristic of the laser diode tends to deviate from a desired input-output characteristic for its normal operation when the temperature of the laser diode rises.
It is necessary to always supply a bias current equivalent to the threshold current to the laser diode, in order to avoid the continuous oscillation mentioned above. Also, in order to maintain the desired input-output characteristic for the normal operation, it is necessary to control the temperature of the laser diode to be constant by making use of a Peltier device or some other device. The Peltier device is a thermoelectric device which is capable of controlling the temperature of the laser diode to be constant by utilizing the Peltier effect.
However, there still remains a problem that an optical transmitter including a laser diode module using the Peltier device for controlling the temperature of a laser diode becomes large in size, and the optical transmitter with such a temperature control device requires a larger amount of power than an optical transmitter without the temperature control device.
FIG. 1 shows a conventional optical transmitter 10. In FIG. 1, the optical transmitter 10 includes an input terminal 11, an output terminal 12, a driving unit 13, a laser diode module (LD MOD) 14, an automatic temperature control unit (ATC) 15, and an automatic power control unit (APC) 16. An electrical input signal which indicates data to be transmitted is supplied to the input terminal 11. A light signal is output from the output terminal 12, and it is transmitted to an external device via an optical transmission line. The laser diode module 14 includes a laser diode which outputs the light signal.
The driving unit 13 drives the laser diode module 14 in accordance with the input signal which indicates digital data including a sequence of values of zeros and ones. The driving unit 13 supplies a pulsed current I.sub.p to the laser diode, the pulsed current I.sub.p being in accordance with the input signal. Also, the driving unit 13 controls the amplitude of the pulsed current I.sub.p in accordance with a pulsed-current control signal output from the APC 16.
A bias current I.sub.b which is equivalent to a threshold current of the laser diode is also supplied to the laser diode. The pulsed current I.sub.p is superimposed over the bias current Ib, and the resultant current is supplied to the laser diode.
The APC 16 monitors a power of the light output by the laser diode. The APC 16 supplies the pulsed-current control signal to the driving unit 13 and supplies a bias-current control signal to the laser diode in accordance with the monitored power. This control procedure is carried out by the APC 16 so as to control the power of the light output by the laser diode to be constant.
The ATC 15 includes a Peltier device (not shown in FIG. 1) which is capable of controlling the temperature of the laser diode included in the LD MOD 14. The ATC 15 controls the Peltier device so that the temperature of the laser diode is controlled to be constant.
FIGS. 2A and 2B show the laser diode module 14 of the conventional optical transmitter 10 in FIG. 1. In FIGS. 2A and 2B, the laser diode module 14 includes a case 17 which hermetically seals the laser diode. A Peltier device 23 is attached to a bottom portion of the case 17. A metal base 19 is attached to a top surface of the Peltier device 23.
A photodetector (PD) 20, a chip carrier 21, and a thermistor 25 are arranged on a top surface of the metal base 19. A laser diode chip 26 is placed on the chip carrier 21. The photodetector 20 monitors a power of the light output by the laser diode chip 26. The thermistor 25 detects a temperature of a peripheral portion of the laser diode chip 26. The detected temperature from the thermistor 25 is used by the Peltier device 23 to control a temperature of the laser diode chip 26 to be constant.
In addition, in the laser diode module 14, an optical unit 22 including a lens and an isolator is arranged between the laser diode chip 26 and an optical fiber line 24. This optical fiber line 24 serves as an optical transmission line through which the light signal from the laser diode module 14 is transmitted. Further, in the laser diode module 14, a number of connection terminals 18 are arranged on the periphery of the case 17. The connection terminals 18 of the laser diode module 14 are connected to the driving unit 13, the automatic temperature control unit 15, and the automatic power control unit 16 which are shown in FIG. 1.
FIG. 3 shows an output portion of the driving unit 13 and the laser diode module (LD MOD) 14 in the conventional optical transmitter 10 in FIG. 1.
In FIG. 3, the laser diode module 14 includes a laser diode LD, a photodetector PD which is the photodetector 20 in FIGS. 2A and 2B, and a damping resistor Rd. A source power line of a source voltage Vcc (the actuated voltage level) is connected to an anode of the laser diode LD. One end of the damping resistor Rd is connected to a cathode of the laser diode LD. The damping resistor Rd is provided to shape a waveform of the light signal and reduce an interference of data of the light signal when the data transmission rate is higher than a gigabit per second.
The light signal from the laser diode LD is output to the optical fiber line 24 via the optical unit 22 (not shown in FIG. 3). The source power line of Vcc is connected to a cathode of the photodetector PD. The photodetector PD detects a power of the light output by the laser diode LD, and supplies a light-output detection signal to the automatic power control unit 16 (shown in FIG. 1) in accordance with the detected power.
In FIG. 3, the output portion of the driving unit 13 includes two transistors TR1 and TR2 which constitute a differential amplifier, and a transistor TR3 and a resistor R1 which constitute a constant-current source. The input signal which indicates a complementary form of data is supplied to each of bases of the transistors TR1 and TR2. The source voltage Vcc (which is, for example, +5 V) from the source power line of Vcc is supplied to a collector of the transistor TR1. A collector of the transistor TR2 is connected to the other end of the damping resistor Rd in the laser diode module 14.
A collector of the transistor TR3 is connected to each of emitters of the transistors TR1 and TR2. The pulsed-current control signal from the APC 16 shown in FIG. 1 is supplied to a base of the transistor TR3. This pulsed-current control signal is a voltage signal which sets a voltage of the base of the transistor TR3. By setting the voltage of the base of the transistor TR3, the pulsed current flowing in the transistor TR3, which is the same as that supplied to the laser diode LD, is controlled. A source power line of a source voltage Vee (the ground level) is connected to an emitter of the transistor TR3 via the resistor R1. The ground-level voltage Vee from the source power line is always lower than the actuated-level voltage Vcc from the source power line.
To supply the bias current Ib, which is equivalent to the threshold current of the laser diode LD, to the laser diode LD, a transistor TR4, an inductor L1, and a resistor R2 are provided in the circuit shown in FIG. 3. A collector of the transistor TR4 is connected to the above-mentioned other end of the damping resistor Rd via the inductor L1 and to the collector of the transistor TR2 via the inductor L1. An emitter of the transistor TR4 is connected to the source power line of Vee via the resistor R2. The bias-current control signal from the APC 16 shown in FIG. 1 is supplied to a base of the transistor TR4. This bias-current control signal is a voltage signal which sets a voltage of the base of the transistor TR4.
FIG. 4 shows an operation of the circuit of the conventional optical transmitter 10 in FIG. 3. In FIG. 4, the abscissa of each chart indicates the input current flowing in the laser diode LD, and the ordinate thereof indicates the power of the light output by the laser diode LD. The threshold current of the laser diode LD is indicated by "Ith" in FIG. 4. In FIG. 4, there are shown some input-output characteristic lines of the laser diode LD with different rates of change of the quantum efficiency which are varied depending on the temperature of the laser diode LD. The bias current I.sub.b is the same as the threshold current "Ith", and the pulsed current I.sub.p is superimposed over the bias current Ib. The amplitude of the pulsed current I.sub.p is set so that the power of the light output follows a corresponding temperature characteristic line.
As shown in FIG. 4, when the temperature of the laser diode LD rises, the threshold current "Ith" increases and the rate of change of the quantum efficiency decreases. The power of the light output by the laser diode is varied depending on the change in the temperature of the laser diode. In other words, when the temperature of the laser diode LD rises, both the bias current I.sub.b and the pulsed current I.sub.p are increased, and the power of the light output by the laser diode is increased.
In order to avoid the increase of the power of the light output by the laser diode, the temperature of the laser diode LD is controlled to be constant by using the Peltier device 23 in the conventional transmitter. In other words, in the conventional transmitter, the temperature of the peripheral portion of the laser diode chip 26 is measured by using the thermistor 25. The measured temperature is used by the ATC 15 to control the Peltier device 23, thereby controlling the temperature of the laser diode LD to be unchanged. Thus, the occurrence of the undesired input-output characteristic of the laser diode is prevented.
Although the temperature of the laser diode can be controlled to be constant by using the Peltier device 23, there is a problem that the laser diode module 14 including the Peltier device 23 becomes large in size, and the automatic temperature control unit 15 of the optical transmitter requires a large amount of additional power for the temperature control operation. Therefore, it is desirable that the size of the optical transmitter be made smaller and the consumption power of the optical transmitter be reduced as much as reasonably possible. It is conceivable to eliminate the Peltier device 23 and the automatic temperature control unit 15 from the optical transmitter, in order to resolve the above problem.
However, if an optical transmitter in which only the Peltier device is eliminated is used, the output of the transistor TR2 of the output portion of the driving unit 13 may be saturated when the temperature of the laser diode rises. The switching of the transistor TR2 in response to the input signal cannot be performed, and it is difficult to correctly supply the pulsed current to the laser diode.